The present invention relates to laser diode assemblies (laser diode modules) which combine output beams emitted by single-emitter laser diode chips.
Diode lasers are compact, inexpensive to manufacture, and easy to service compared to many other types of lasers. An individual laser diode has a fairly low output power (typically from hundreds of milli-Watt to several Watt), but laser diodes can be combined to obtain a high-power beam, e.g. in kilo-watt range. When laser diode beams are combined, the combined beam should preferably be high in brightness and power, and have near diffraction-limited beam quality. However, beam combining for laser diodes is challenging because an individual diode's output beam is asymmetric, being elongated along the diode's pn junction. The beam's divergence is also asymmetric—the divergence is lower along the “slow” axis parallel to the pn junction than along the “fast” axis perpendicular to the pn junction. (The beam divergence along the slow axis can be about 6°-10° measured Full Width at Half Maximum, and along the fast axis about 30°-40°.) This asymmetry complicates beam-combining optics. Complex optics can reduce output brightness and power. In addition, the small size of laser diodes makes cooling more difficult. Uniform cooling is important to reduce mechanical stresses and to maintain alignment between the laser diodes and the optics (misalignment leads to loss of output brightness and power). Cooling is also important for controlling the diodes' output spectrum since the central wavelength and spectral width increase with temperature. Further, heat causes degradation of the diodes' slope efficiency (i.e. the efficiency in converting electrical energy to light).
A multi-diode system can be formed by manufacturing a multiple-emitter chip, e.g. a laser bar or stack, with multiple diodes in a single semiconductor chip. High output power, of tens and even hundreds of watts, can be achieved. However, multiple-emitter diode chips are less reliable than single-emitter diode chips because the multiple-emitter diode chips are harder to cool in uniform manner. Also, a diode's failure in a multiple-emitter chip can affect other diodes in the chip, so the entire chip can become unusable if a single diode fails. Therefore, multi-diode systems of single-emitter chips remain attractive despite their larger size.
FIGS. 1A (top view), 1B (front view), 1C (three-dimensional view) illustrate one such system disclosed in U.S. patent application Ser. No. 12/116,834 filed by Wilson et al., published as US 2008/0310027 A1 on Dec. 18, 2008. The system combines output beams 112 of seven single-emitter laser diode chips 110.1-110.7 to produce a combined beam 114, which is then focused by lens 118 into optical fiber 122. Each diode's output beam 112 is collimated in the fast axis by a respective fast-axis collimator (SAC) 130. The diodes share slow-axis collimator (SAC) 134. The output beam of diode 110.7 is collimated by respective FAC 130 and is then delivered directly to SAC 134. The other six diodes 110.1-110.6 emit beams perpendicular to the output beam of diode 110.7. The output beams of diodes 110.1-110.6 are collimated by the respective FACs 130, and are then reflected (folded) by respective beam-redirecting mirrors 140 towards SAC 134. The seven diodes 110, fast-axis collimators 130, and mirrors 140 are arranged so that their respective beams 112 are stacked one above another (see FIG. 1B) at the input of SAC 134. More particularly, the diodes 110.1-110.7 are arranged at respective different heights, i.e. with a step along the Y direction. Laser diode 110.7 emits the top beam 112. The remaining diodes 110.1-110.6 are arranged in pairs. In each pair, the diodes emit beams in the opposite directions, at different heights. The two mirrors 140 in each pair are located on top of each other. The diodes are located at different distances from the optical axis of the combined beam 114 to equalize the distances between the laser diodes' emitters and the combined beam 114.
As shown in FIG. 1C, the system includes a package 170 which serves as a compound heat sink for the laser diode chips 110. Mirrors 140 are shown as three X-like structures. The mirrors and the chips 110 are disposed with a vertical step (along the Y direction) of 1 mm.
FIGS. 2A-2F show an assembly described in U.S. Pat. No. 7,420,996 B2 issued Sep. 2, 2008 to Schulte et al. This assembly combines multiple single-diode-chip subassemblies 204 one of which is illustrated in FIGS. 2A-2C. Each subassembly 204 includes a single emitter diode laser 110, a mounting block 210, a submount 214, and a FAC lens 130. The subassembly also includes a SAC lens 134, but this lens is used to collimate the output of a laser 110 of the adjacent subassembly 204 as shown in FIG. 2D.
Diode laser 110 is mounted on submount 214 attached to mounting block 210. Submount 214 includes contact pads for contacting the diode laser 110. Submount 214 and mounting block 210 have high thermal conductivity.
Each subassembly 204 is mounted on a respective step 220 (FIG. 2E) of cooling block 224. The subassembly's mounting block 210 provides a thermal path to cooling block 224.
As shown in FIGS. 2D-2F, cooling block 224 holds a row of subassemblies 204 held in place by claims 226. In each subassembly 204, the beam 112 emitted by diode laser 110 is collimated by FAC 130 of the same subassembly, and then by SAC 134 of the next subassembly on the adjacent, lower step 220. For the last, lowest diode 110, the SAC 134 is located on a separate stand 250. The diodes' output beams 112 are stacked one above another in the combined beam, like in the embodiment of FIGS. 1A-1C.
Cooling block 224 can be widened to hold two rows of subassemblies 204, with five diodes in each row. The assembly then produces two combined output beams, which can be combined using known optical techniques.